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CHAPTER 2

Network Models

Review Questions 1. List the layers of the Internet model.

Ans: The layers of TCP or Internet model are:

i. Physical layer ii. Data link layer

iii. Network layer iv. Transport layer and v. Application layer.

2. Which layers in the Internet model are the network support layers?

Ans: Physical, data link, and network layers.

3. Which layer in the Internet model is the user support layer?

Ans: In the Internet Model only Application layer supports the user.

4. What is the difference between network layer delivery and transport layer delivery?

Ans: The transport layer is responsible for process-to-process delivery of the entire message, but

the network layer oversees host-to-host delivery of individual packets.

5. What is a peer-to-peer process?

Ans: Peer-to-peer processes are processes on two or more devices communicating at a same

layer

6. How does information get passed from one layer to the next in the Internet model?

Ans: Each layer calls upon, the services of the layer just below it using interfaces between each

pair of adjacent layers.

7. What are headers and trailers, and how do they get added and removed?

Ans: Headers and trailers are control data added at the beginning and the end of each data unit at

each layer of the sender and removed at the corresponding layers of the receiver. They provide

source and destination addresses, synchronization points, information for error detection etc.

8. What are the concerns of the physical layer in the Internet model?

Ans: The physical layer is responsible for transmitting bits. Also it is concerned with

a. physical topology

b. representation of bits

c. type of encoding

d. synchronization of bits

e. transmission rate and mode

f. Line Configuration

9. What are the responsibilities of the data link layer in the Internet model?

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Ans: The data link layer is responsible for

a. Framing data bits

b. Physical addressing

c. Data rate means flow control

d. Error Control

d. Access Control

10. What are the responsibilities of the network layer in the Internet model?

Ans: The network layer is responsible for transmitting of a packet across multiple networks;

therefore its responsibilities include

a. Logical addressing

b. Routing

11. What are the responsibilities of the transport layer in the Internet model?

Ans: The transport layer oversees the process-to-process delivery of the entire message. It is

responsible for

a. Segmentation and reassembly

b. Connection Control

c. Flow control

d. Error control

12. What is the difference between a port address, a logical address, and a physical address?

Ans: The physical address is the local address of a node; it is used by the data link layer to

deliver data from one node to another within the same network. The logical address defines the

sender and receiver at the network layer and is used to deliver messages across multiple

networks. The port address (service-point) identifies the application process on the station.

13. Name some services provided by the application layer in the Internet model.

Ans: The application layer services include file transfer, remote access, shared database

management, and mail services. Also responsible for dialog control, synchronization, (concern

session layer in OSI model) translation, encryption and compression. (concern presentation layer

in OSI model)

14. How do the layers of the Internet model correlate to the layers of the OSI model?

Ans: The application, presentation, and session layers of the OSI model are represented by the

application layer in the Internet model. The lowest four layers of OSI correspond to the Internet

model layers.

Exercises

15. How are OSI and ISO related to each other?

Ans: The International Standards Organization, or the International Organization of Standards,

(ISO) is a multinational body dedicated to worldwide agreement on international standards. An

ISO standard that covers all aspects of network communications is the Open Systems

Interconnection (OSI) model.

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16. Match the following to one or more layers of the OSI model: a. Route determination

b. Flow control

c. Interface to transmission media

d. Provides access for the end use

Ans:

a. Route determination: network layer

b. Flow control: data link and transport layers

c. Interface to transmission media: physical layer

d. Access for the end user: application layer

17. Match the following to one or more layers of the OSI model: a. Reliable process-to-process message delivery

b. Route selection

c. Defines frames

d. Provides user services such as email and file transfer

e. Transmission of bit stream across physical medium

Ans: a. Reliable process-to-process delivery: transport layer

b. Route selection: network layer

c. Defining frames: data link layer

d. Providing user services: application layer

e. Transmission of bits across the medium: physical layer.

18. Match the following to one or more layers of the OSl model:

a. Communicates directly with user's application program

b. Error correction and retransmission

c. Mechanical, electrical, and functional interface

d. Responsibility for carrying frames between adjacent nodes

Ans: a. Communication with users application program: application layer b. Error correction and retransmission: data link and transport layers

c. Mechanical, electrical, and functional interface: physical layer

d. Responsibility for carrying frames between adjacent nodes: data link layer

19. Match the following to one or more layers of the OSI model:

a. Format and code conversion services

b. Establishes, manages, and terminates sessions

c. Ensures reliable transmission of data

d. Log-in and log-out procedures

e. Provides independence from differences in data representation

Ans:

a. Format and code conversion services: presentation layer

b. Establishing, managing, and terminating sessions: session layer

c. Ensuring reliable transmission of data: data link and transport layers

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d. Log-in and log-out procedures: session layer

e. Providing independence from different data representation: presentation layer.

20. In Figure 2.22, computer A sends a message to computer D via LANl, router Rl, and LAN2.

Show the contents of the packets and frames at the network and data link layer for each hop

interface.

Ans:

21. In Figure 2.22,(given avobe) assume that the communication is between a process running at

computer A with port address i and a process running at computer D with port address j. Show

the contents of packets and frames at the network, data link, and transport layer for each hop.

Ans:

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22. Suppose a computer sends a frame to another computer on a bus topology LAN. The physical

destination address of the frame is corrupted during the transmission. What happens to the

frame? How can the sender be informed about the situation?

Ans: If the corrupted destination address does not match any station address in the network, the

packet is lost. If the corrupted destination address matches one of the stations, the frame is

delivered to the wrong station. In this case, however, the error detection mechanism, available in

most data link protocols, will find the error and discard the frame.

23. Suppose a computer sends a packet at the network layer to another computer somewhere in

the Internet. The logical destination address of the packet is corrupted. What happens to the

packet? How can the source computer be informed of the situation?

Ans: Before using the destination address in an intermediate or the destination node, the packet

goes through error checking that may help the node find the corruption (with a high probability)

and discard the packet. Normally the upper layer protocol will inform the source to resend the

packet.

24. Suppose a computer sends a packet at the transport layer to another computer somewhere in

the Internet. There is no process with the destination port address running at the destination

computer. What will happen?

Ans: Most protocols issue a special error message that is sent back to the source in this case.

25. If the data link layer can detect errors between hops, why do you think we need another

checking mechanism at the transport layer?

Ans: The errors between the nodes can be detected by the data link layer control, but the error at

the node (between input port and output port) of the node cannot be detected by the data link

layer.